Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of next_thread()

Kui-Feng Lee
2023-08-21T17:56:04Z

On 8/21/23 08:09, Oleg Nesterov wrote:
> 1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we
> can safely iterate the task->thread_group list. Even if this task exits
> right after get_pid_task() (or goto retry) and pid_alive() returns 0 >
> Kill the unnecessary pid_alive() check.

This function will return next_task holding a refcount, and release the
refcount until the next time calling the same function. Meanwhile,
the returned task A may be killed, and its next task B may be
killed after A as well, before calling this function again.
However, even task B is destroyed (free), A's next is still pointing to
task B. When this function is called again for the same iterator,
it doesn't promise that B is still there.

Does that make sense to you?

>
> 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
> check.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> kernel/bpf/task_iter.c | 7 -------
> 1 file changed, 7 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index c4ab9d6cdbe9..4d1125108014 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> return NULL;
>
> retry:
> - if (!pid_alive(task)) {
> - put_task_struct(task);
> - return NULL;
> - }
> -
> next_task = next_thread(task);
> put_task_struct(task);
> - if (!next_task)
> - return NULL;
>
> saved_tid = *tid;
> *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);